1. Field of the Invention
The present invention relates to curing photo-curable dental compositions. In particular, the present invention provides a method to cure dental compositions using a light-emitting diode (LED) as a source of curing radiation placed proximate to the composition to be cured.
2. Description of the Related Art
Certain polymeric materials useful in the field of dentistry for adhesion, sealing and restoration may be cured or hardened upon exposure to a source of radiation. Such photoactive materials are known as "photo-curable dental compositions" and generally harden when exposed to radiation having wavelengths in the visible range. Photo-cured dental compositions are convenient for use by a dentist because the curing process can be initiated when the dental composition has been accurately placed in its proper position. A source of radiation energy positioned proximate to the material to be hardened, for example an appropriate amount of composition placed inside a tooth cavity, is activated to initiate polymerization and subsequent curing of the composition to secure the repair.
Photo-cured dental compositions were initially hardened by the application of concentrated beams of ultraviolet (UV) radiation. In order to provide such UV radiation, dental guns and other apparatuses for producing concentrated beams of UV radiation were developed. See U.S. Pat. Nos. 4,112,335 and 4,229,658, for example. Later, visible light curable dental compositions were used and dental radiation guns for producing concentrated visible light were provided like that that disclosed in U.S. Pat. No. 4,385,344. However, a relatively high divergence about 25 degrees of the light beam from such visible light sources reduces penetration into the tooth structure, leading to their relative inefficiency and unreliability for photo-curing dental composition that are thicker than about two millimeters.
Photo-curable dental materials have also been developed that are hardened by exposure to radiant energy in a preselected spectral range. Typically, a photo-activated chemical reaction in many photo-curable dental materials is initiated by application of a high intensity blue light having a wavelength of 400-500 nanometers. Since the light sources employed typically produce the entire visible light spectrum as well as some non-visible radiation, a reflector is coated to reflect only visible light, and the filters are selected to substantially block non-visible radiation and visible light other than blue light in the range of 400-500 nanometers, in order to produce the desired range of radiation, as shown for example in U.S. Pat No. 5,147,204. Laser-based radiation sources have also been employed, using for example, a Nd YAG laser producing radiation at a wavelength of about 1060 nanometers, in combination with a frequency doubling material as disclosed for example in U.S. Pat. No. 5,885,082. In the instance that a laser source is used, the beam must be de-focussed to cover the area being cured and this is done by varying by hand the distance between the dental composition and the laser dental gun.
There are several disadvantages in using light curing apparatuses of the prior art like those discussed above. Commercially available dental light guns often include an elongated, slender light guide such as a bundle of optical fibers having a free end that can be positioned close to the photo-curable material in order to direct light to the material from a light source located outside the oral cavity. Thus, because of the relatively large size of the dental gun within a patient's mouth, a degree of physical discomfort is introduced to the patient as well as to the dentist who must hold the gun steady for about one minute.
Second, the area illuminated by conventional blue-filtered metal-halide radiation is usually in the range of about a 1/2-inch diameter circle and over a typical curing cycle of about 60 seconds. The relatively high energy output and beam divergence of such dental guns leads to the possibility of increased heating of the pulp tissue which is sensitive to small changes in temperature.
In addition, when dental compositions are cured in place within a cavity for instance, after curing an amount of shrinkage of about 2.5% occurs leaving a gap within the area being treated; such shrinkage is so deleterious that any small reduction in shrinkage is desirable.
Furthermore, in tests of cure depth uniformity of standardized compositions, it was found that a high percentage (46%) of curing lights used in private dental offices are unsuitable for use when tested against manufacture's recommendations using a curing radiometer or a heat radiometer, due in part to the loss of output of the light source in use [J Dent March199 ;27(3):235-41]. Finally, due to the expenses of combining a laser or metal-halide radiation source, focussing elements, power sources, etc., significant expense are involved in purchasing and using dental guns. Conventional dental curing devices are therefore seen to have shortcomings including uncomfortable use, unreliable curing and relatively high expense.
U.S. Pat. No. 4,385,344 discloses a dental gun device for production of light in the low visible range for photo-curing dental compositions, the device comprising a tungsten halogen lamp with a concentrating reflector which reflects visible light and passes middle and far infrared wavelengths. A dichroic heat reflecting filter which passes light from 400 to 700 nm and reflects energy in the visible red and near infrared wavelengths back to the lamp envelope, enhances lamp halogen cycle efficiency. The dichroic heat reflecting filter is followed by a dielectric filter which provides a high efficiency bandpass at the desired visible range. A fiber optic light guide is positioned to receive the focused and filtered light and to transmit it to a reduced surface light applying tip at the end of the handpiece. The fiber light guide is encased in a specially designed sheathing which provides protection to the optical fibers and carries two electrical conductors which are connected between a control switch on the handpiece and the power supply for the lamp.
U.S. Pat. No. 5,147,204 is representative of conventional blue-light filtered dental guns. This patent discloses a blue light emitting apparatus for curing photo-curable dental material including a handpick having a housing, a depending handle and a detachable light guide. The light guide is received in a head connected to the housing. A source of tungsten-halogen light is coupled to the housing, and a light guide is detachably connected to the head for communication with the source of light. Since the tungsten-halogen light produces the entire visible light spectrum as well as some non-visible radiation, a reflector is coated to generally reflect only visible light, and a blue-pass filter and a heat filter are selected to substantially block non-visible radiation and visible light other than blue light in the range of 400-500 nanometers.
U.S. Pat. No. 5,885,082 is representative of the use of pulsed laser radiation having a selected wavelength for performing a variety of dental procedures, including curing and hardening of a dental composition containing hydroxyapatite and phosphoric acid. The use of laser radiation for curing employs a housing provided with an optical fiber coupled to a source of monochromatic light, such as an Nd YAG laser operating at a wavelength of 1060 nanometers. The optical fiber directs light radiation onto a curved mirror which deflects the radiation onto the receiving end of a further optical fiber. A frequency doubling material influences the laser radiation so that such a laser-based dental gun has the capability of applying either 1060 nanometers or 532 nanometers radiation to the area to be treated. It is significant that cooling water is disclosed as being sprayed onto the tooth in the vicinity of the spot which is being irradiated, especially when radiation at 532 nanometers is applied. A further disadvantage of the use of laser radiation is de-focussing the laser beam to be coextensive with the surface of filling composition by varying by hand the spacing between the laser dental gun and the tooth surface.
Accordingly, from a study of the different approaches taken in the prior art to the problems presented by the necessity for reliably providing a minimum essential amount of curing radiation without undue heating or other discomfort to a patient or dentists, there remains a need for an improved approach to dental polymer curing that is compatible with existing polymeric compositions and which involves smaller, easier to use and less expensive devices that operate with a smaller amount of applied radiation energy, yielding lower temperatures, and less composition shrinkage. A further need is the availability of a light source that has insignificant loss of output during use.